The present invention relates to an antiskid control device which prevents loss of directional stability or controllability of a motor vehicle due to locking of wheels at the time of braking and ensures a minimum braking distance.
Conventionally, in control of a braking pressure in antiskid control devices for motor vehicles, it has been generally so arranged that when locking of wheels has been detected at the time of braking on the basis of a slip speed identical with a difference between a vehicle speed and a wheel speed, a braking pressure is reduced so as to recover the wheel speed to such a level as to obtain a proper slip speed such that not only locking of wheels is prevented but a braking distance is minimized.
However, the known antiskid control devices have such a drawback that when the braking pressure has been reduced upon detection that the wheels trend towards locking, the braking pressure may be frequently reduced excessively owing to methods of decision of pressure reduction or delay in actuation of a braking system. In such a case, since a braking force becomes too small, rotation of the wheels recovers rapidly beyond a proper slip state up to a level approximate to the vehicle speed and thus, the braking distance is increased if no countermeasure is taken.
Thus, if a command of increasing the braking pressure is issued upon detection of recovery of rotation of the wheels, the proper slip state is brought about at last. At this time, in the known antiskid control devices, a state of the wheels trending towards locking again in response to increase of the braking pressure after recovery of the wheels from locking is detected by such a criterion that an acceleration of the wheels has dropped below a predetermined acceleration reference value .alpha.1 into a deceleration region. Meanwhile, the acceleration of the wheels may drop below the predetermined acceleration reference value .alpha.1 into the deceleration region also while the wheels whose rotation has recovered to the vehicle speed are being shifted to the proper slip state. Thus, in this case, since it is decided that the wheels have trended towards locking, antiskid control such as pressure reduction against a symptom of locking of the wheels is performed. As a result, such an inconvenience is incurred that the braking pressure cannot be increased at the time when the braking pressure is actually required to be increased so as to recover a braking force.
As described above, such an undesirable phenomenon takes place after the wheels have been recovered from locking rapidly due to the excessively small braking force. Thus, in order to eliminate the undesirable phenomenon, a method is considered in which when the wheels are accelerated beyond a predetermined acceleration reference value .alpha.2 at the time of recovery of the wheels from locking, new detection of locking of the wheels is subsequently prohibited during a predetermined time period. However, this method is also disadvantageous in that even when the wheels have actually trended towards locking again due to an exceedingly large value of the braking pressure after rapid recovery of rotation of the wheels, a control action for avoiding locking of the wheels cannot be started until the predetermined time period elapses.